Fiber optic communication systems offer far greater capacity than their electrical counterparts (e.g., twisted pair, or coaxial cable) and are therefore attracting much attention as the number and complexity of bandwidth-intensive applications increase. This ever-increasing need for bandwidth that only fiber can deliver is resulting in the continued, widespread deployment of fiber networks.
Legacy fiber systems have for many years existed primarily as untapped, long-haul, point-to-point links between “central offices.” But simple market forces are pushing fiber networks beyond central offices and into the more architecturally diverse terrain of “metro” markets.
In existing systems, a single wavelength band carrying a single modulated data stream is transmitted across a single fiber link. Digital time division multiplexing (TDM) of the data stream can be used to accommodate separate, independent data channels over the same wavelength band, but these systems require expensive up/down-conversion of the optical signal to an electrical version for multiplexing/demultiplexing the separate channels. While providing some level of operational channelization, TDM techniques generally do not increase the overall data capacity of a single wavelength link.
Dense wavelength division multiplexing (DWDM) enables the transmission of multiple, independent wavelength channels across a single fiber, thus providing some channelization and a much greater data capacity. Predictably, this capability has resulted in the requirement to add or drop these wavelength channels along the previously untapped lengths of fiber to provide access to the individual wavelength channels. Optical add/drop multiplexers (OADMs) are employed for this function, enabled by arrayed waveguide grating (AWG) demultiplexers for filtering and forwarding individual wavelengths from a multiplexed stream; or AWG multiplexers for combining multiple, individual wavelengths into a multiplexed stream.
AWGs are in the class of “integrated” wafer-based optical components, called planar lightwave circuits (PLCs). AWGs can be useful in many optical communication applications where wavelength-specific filtering and processing are required. Unlike the legacy TDM systems, AWGs function purely in the optical domain when filtering the independent wavelength bands and thus do not require expensive, electrical up/down conversion. As all-optical wavelength filtering components, AWGs have become attractive for optical communication systems.
For example, with reference to the functional schematic of FIG. 1a, a conventional demultiplexer 10 has a single multiplexed input optical signal 12 including channels centered at wavelengths λ1-40 (e.g., 100 GHz spacing and 25 GHz pass bands for one known communication system). Individual demultiplexed channels centered at respective wavelengths λ1 . . . λ40 are outputs of this demultiplexer, each on its own respective output port 14.
FIG. 1b depicts an exemplary AWG embodiment of demultiplexer 10. The AWG includes an array of closely spaced array waveguides 22 having carefully controlled, differing path lengths which cause constructive phase interference patterns on the optical signals transmitted into the device. As discussed above, this technique is useful for multiplexing or demultiplexing optical signals transmitted from the array input waveguides 24—distributed by planar waveguide region 25 to array waveguides 22—then refocused through the output planar waveguide region 27 to output waveguides 26.
For a 40–48 channel device with 100 GHz spacing, a free spectral range of 6400 GHz is preferable (i.e., 64 total channels to remove the effects of outer channel rolloff). However, the channel plan of certain communication systems of interest may offer opportunities to reduce the bandwidth of each mux/demux. For example, with reference to the channel spectrum of FIG. 2, 40 total channels are shown, each at its own respective wavelength λ1 . . . λ40. However, along certain communication paths, this channel plan can be broken into bands. Exemplary separation of this channel plan into five bands, each having 8 channels, is shown. Filtering may only be required within, but not outside, each band, thus reducing the required component bandwidth in these paths.
FIG. 3 depicts one exemplary approach to banded operation, i.e., separate demultiplexers 301 . . . 305 specially designed for each band of interest. This approach, while offering good intra-band performance of each component (due to their narrower bandwidths), requires five separate components, adding to inventory cost and complexity.
Other types of OADM components, for example those based on thin film filters, suffer a similar disadvantage: each filter is designed for a specific wavelength and therefore separate components are needed for each band.
The requirement of separate components for each band has non-trivial implications. Separate sets of technical specifications are required, as are separate procurement channels and inventory requirements.
What is required, therefore, are improved component design and packaging techniques which capitalize on banded operation but which do not require the conventional, separate technical and procurement specifications.